Motor vehicles (cars and trucks) use energy to travel, mostly to overcome friction between the tires and the road and air friction with the body of the vehicle, but also to push air out of the vehicles' way as it moves forward.
Vehicle bodies have been made increasingly aerodynamic to reduce the force required to push them rapidly forward through the air, but there are inherent limitations to what can be done in this regard. Such limitations are due to the practical requirements of providing an adequate interior space in the vehicle for occupants and providing an adequate view forward for the driver.
A major obstacle to aerodynamic design is the vehicle windshield. The glass plate forming this part of the vehicle cannot be slanted much more than about 45 degrees to the vertical or the view of the road ahead becomes distorted.
As a result, the rate of energy consumption of the vehicle, measured in miles per gallon of fuel consumed, increases as the speed of the vehicle is increased beyond a given cruising speed.
According to studies backed by the U.S. Department of Energy, the fuel consumption of the average car peaks at about 55 MPH. As the speed increases beyond 55 MPH the vehicle is:                3% less efficient at 60 mph,        8% less efficient at 65 mph,        17% less efficient at 70 mph,        23% less efficient at 75 mph, and        28% less efficient at 80 mph.        
At such higher speeds the vehicles waste gas and create unnecessary greenhouse gases.